Volcanic sequences exposed on the Tonga forearc and trench slope were investigated using a submersible to document seafloor outcrops and characterise volcanic lithofacies. These observations were used to establish lithofacies relationships and interpret depositional environments and volcanic processes. At similar to 5080 m below sea level, the sequences record both effusive and explosive volcanism, including lava flows, pumice fall and pyroclastic density current deposits. When integrated with over 70 years of regional geological data, these lithofacies are interpreted as Eocene subaerial rhyolitic volcanism, forming part of the forearc basement. Visual observation of seafloor exposures can help ground-truth, bridging the gap between sampling biases of dredging, the limited spatial coverage of drilling campaigns, and large-scale geophysical surveys. The data provide new documentation of volcanic lithofacies across previously inaccessible depths of the Tonga forearc and trench, offering rare insights into volcanic deposits that may otherwise be poorly preserved in the geological record or difficult to recover. The presence of subaerially derived volcanic sequences at these depths supports non-accretionary forearc models and highlights the value of targeted submersible investigations.

Abstract Pyroclastic density currents represent one of the deadliest hazards posed by active volcanoes. Analysis of their deposits provides valuable insights into their internal dynamics and informs numerical simulations of pyroclastic density currents which underpin many volcanic hazard assessments. We present PDCD-DAT, a global database of pyroclastic density current deposit characteristics compiled from peer-reviewed literature. The database includes both quantitative datasets (e.g., grain size, density, bedform dimensions, thickness) and qualitative descriptors (e.g., sedimentary structures, lithofacies). PDCD-DAT includes data from 85 source publications, covering 97 eruptions or eruptive phases, and 214 individual depositional units from 55 globally distributed volcanoes. Eruptions recorded in the database range from VEI 1–8. We highlight examples of potential applications of the database, which include (i) comparison of single deposit case studies to global datasets, (ii) informing input parameters and conditions for numerical and analogue models of pyroclastic density currents, (iii) validation of numerical and analogue models against a wide variety of natural case study deposit architectures, and (iv) estimating hazard impact metrics of pyroclastic density currents from past eruptions. We show that the database represents a useful tool for improving our ability to model pyroclastic density currents, predict their associated hazards, and understand the relationships between the internal dynamics of pyroclastic density currents and the properties of their deposits. PDCD-DAT is integrated with the FlowDat Mass Flow Database, which provides a sustainable platform for the database. We aim for PDCD-DAT to be expanded in the future through addition of pyroclastic density current deposit datasets from new field studies conducted by the volcanology research community.

Abstract The ability to rapidly disseminate scientific knowledge across the world has never been greater. This is paralleled by the ability of rumors to spread equally far and fast. Popular misconceptions can take root and, without correction, take on the appearance of facts. Commonly misunderstood topics include those with attention-grabbing phrasing (e.g. “Pacific Ring of Fire”, “supervolcanoes”, or “mega-tsunamis”), the intersection of volcanic research with high-profile science topics (climate or human health), aspirational science goals (eruption forecasts), and basic terminology (volcanic ash versus smoke, or magma “chambers”). In this paper, we describe eighteen misconceptions grouped in six themes (seismology, volcanoes and climate, systems and structures, volcanic hazards, timescales, photographs and footage) commonly encountered by volcanologists who are actively engaging with the public and media both during and between eruptions. Each misconception description is accompanied by takeaway points and summaries of recent research to assist the reader with the information necessary to describe the issues. Finally, we present seven case studies where incorrect information was widely shared and counteracted at well-known volcanoes and recent eruptions at Mount Agung (Bali, Indonesia), Mount Etna (Sicily, Italy), Kīlauea (Hawaii, U.S.A.), Tajogaite (La Palma, Spain), La Soufrière (St. Vincent), Taal Volcano (Philippines), and Yellowstone caldera (U.S.A.). This work is intended to help both volcanologists and non-specialists get the story straight, and to effectively communicate processes and scales of volcanism and volcanic hazards, impacts, and risks.

Pyroclastic Density Currents (PDCs) are hazardous, multiphase currents of heterogeneous volcanic material and gas. Their high mobility can be partially attributed to fluidisation mechanisms. Moisture (as liquid or gas) can enter a PDC through external (e.g., interaction with bodies of water) or internal (e.g., initial eruptive activity style) processes and presence of moisture can be recorded within distinct deposit layers. We use analogue experiments to explore the behaviour of volcanic material with increasing moisture percentages from 0.00 – 10.00%. Our results show that: 1) the cohesivity of ignimbrite material changes with the addition of small amounts of moisture; 2) Small increases in moisture content change the flow behaviour from a free-flowing material to a non-flowable material; 3) changes in moisture can affect the formation of gas escape structures, and fluidisation profiles, 4) gas flow through a deposit can lead to a moisture profile and resulting mechanical heterogeneity within the deposit and 5) where gas escape structure growth is hindered by cohesivity driven by moisture, pressure can increase and release in an explosive fashion. This work highlights how a suite of dynamic and varied gas escape morphologies can form within the deposit resulting from moisture content heterogeneity, explaining variation in gas escape structures as well as providing a potential mechanism for secondary eruptions.

The Mars Science Laboratory (MSL) Curiosity rover continues to ascend Aeolis Mons in Gale crater, Mars, with the goal of characterising formerly habitable palaeoenvironments. Since September 2022, Curiosity has been traversing Gediz Vallis, a ~9-km long canyon incising into sulfate-bearing, sedimentary rocks on the northern margins of Aeolis Mons. Along the Gediz Vallis floor is the upper Gediz Vallis Ridge (uGVR), a quasi-sinuous, ~1.5 km long, ~80-100 m wide, ~5-30 m high, ridge. Upslope, uGVR is clearly set within an erosional channel, which disappears downslope. Near the Gediz Vallis outlet, uGVR transitions into the broader, lower GVR, recently interpreted by Bryk et al. (2023, AGU Fall Meeting) as a degraded alluvial fan. Since entering Gediz Vallis, Curiosity has undertaken an extensive long-distance imaging campaign of the eastern uGVR flank, acquiring multiple Mastcam and ChemCam Long Distance Remote Micro Imager (LD-RMI) mosaics. Additionally, in August 2023, Curiosity approached the ridge margins and conducted an in-situ investigation (“Region B”). A major objective of Curiosity’s uGVR campaign is to the determine the primary depositional conditions and palaeoenvironment of the ridge, which may record evidence for late-stage surface water flow in Gale. Thus far, most uGVR exposures observed are formed of loosely consolidated, very poorly sorted, decimeter to meter-scale blocks. Most blocks are dark in tone and some are partially embedded within a finer-grained, matrix-like material. The blocks themselves are reworked, lithified, sedimentary rocks and many display a diversity of internal planar and/or cross-stratification, although others appear more massive. Where visible, bedding within the blocks is typically mm to cm in thickness. We used the Pro3D software package to measure mean diameter size of 70 blocks at Region B from stereo Mastcam images: 0.15±0.11 m; although we note that the largest block (~ 7 m diameter) occurs elsewhere. We also note that many blocks are fractured into multiple pieces, potentially due to post-depositional weathering processes. There is no obvious source bedrock from within the ridge that the blocks could be eroding from, consistent with the blocks being transported clasts, rather than erosional lag originating from in-situ bedrock. Generally, the clasts are very angular to sub-rounded, suggesting relatively limited transport and a local source bedrock. The large clast size (cobble, boulder) and very poor sorting is consistent with deposition by debris flows: gravity-driven flows in which clasts are supported by a cohesive muddy matrix. The confinement of the uGVR to a channel argues against a completely unconfined flow, such as a drier landslide, forming the deposit. Post-depositional erosion has likely winnowed much of the finer grained fraction of the deposit. The sedimentary structures within the clasts (e.g., asymptotic cross-stratification) are similar to those in the Stimson formation, an aeolian sandstone, pointing to a potential upslope extension of Stimson, consistent with recent long-distance observations, though multiple sources are also possible. If our interpretation is correct, this would demonstrate that uGVR is providing access to lithologies transported from higher up Aeolis Mons.

Pyroclastic density currents (PDCs) pose substantial risk to populations living on and around active volcanoes, but their structure and internal dynamics are poorly understood. Much of this understanding is derived from interpretation of their widespread deposits. Scaled experiments are able to probe different conditions, to explore how changing flow dynamics relate to the wide variety of depositional styles observed in nature. Here we present two suites of work, first exploring the generation of spontaneous unsteadiness, and how it can impact the partitioning of sediment between dense granular under-currents, and over-riding dilute particle clouds. Second, we introduce grainsize variation to the dense granular regime and explore the formation of grading patterns. We demonstrate that unsteadiness in flow can be important in capturing different grading structures in deposits, and that granular sorting mechanisms are highly effective in thin fluidized grainflow. We conclude that this may raise challenges for the interpretation of common poorly sorted lithofacies (massive lapilli tuff) in natural deposits, as it must require substantial vertical mixing within these grainflows.

When the first transoceanic telegraph cables were laid in the mid-1800s, rapid communication between continents became possible. The advent of fibre-optic submarine cables in the 1990s catalyzed a global digital revolution. Today, a network of > 1.7 million kilometres of fibre-optic cables crosses the oceans, carrying more than 99

The Amapari Marker Band (AMB) is a layer within the Mount Sharp stratigraphy that has been mapped around the Gale crater in orbital images and was recently investigated up close by the Curiosity rover. Symmetric wave ripple marks within the AMB indicate a lacustrine depositional environment in the area investigated along the Curiosity traverse. The wavelength and morphology of the ripples constrain the water depth to a few meters or less. The lateral continuity of the ripple unit defines a minimum extent of the lake during ripple formation. The stratigraphy of the AMB is consistent with an environment of increasing water depth during sedimentation and the lateral correlation of the AMB stratigraphy suggests a transgressive depositional system building upon an eroded surface. The location of the AMB within the surrounding aeolian stratigraphy, coupled with the progression of depositional environments through the Mirador formation, records a pattern of a rising water table relative to sedimentation rates. The potential regional extent of the lacustrine environment, based on orbital mapping of the AMB's variable elevation, spans at minimum 2.0 km of the lateral AMB deposit in the area around Marker Band Valley and may have extended up to 14 km to the west across the northern Gale crater.

The modern surface of Mars does not sustain liquid water, however relict landforms observed on orbital images provide strong evidence of past aqueous activity. Nevertheless on-the-ground analysis of sedimentary strata are required to robustly characterise the specific nature of early Mars palaeoenvironments. The Mars 2020 Perseverance rover is exploring a prominent sedimentary fan deposit at the western margin of Jezero crater – the Western fan – which has been interpreted to be an river delta that prograded into an ancient lake basin during the Late Noachian-Early Hesperian epochs on Mars (~3.6-3.8 Ga). Perseverance’s traverse across the fan in 2022-2023 provides a remarkable window into a fossilised sediment routing system on Mars with potential to understand how water and sediment were distributed across a Martian landscape under a markedly different climate to present day. Here we use the rover’s Mastcam-Z cameras to characterise sedimentary geometries in a distal to proximal transect across the western fan and reconstruct sediment dynamics on the Western fan and infer past environmental change. The distal reaches of the preserved fan show a sedimentary succession that records a transition from distal alluvial fan into lacustrine and subsequently foreset delta deposits. This succession records the initiation of a martian lake system and lake level rise, though the delta stratal geometries suggest deposition during episodes of lake level fall. In the medial sector of the upper exhumed portion of the fan, complex stratal geometries are observed with a variety of scenarios for palaeoenvironmental interactions possible. In particular, the presence of large-scale foreset units preserved in this ‘mid-fan’ sector possibly suggests complex deltaic interfingering with fluvial strata during lake level fluctuations. In more proximal and stratigraphically higher (and hence younger) sectors of the fan, we observe strata deposited by progradation of fluvial systems culminating in a sequence of rounded boulder-containing deposits that signal transition to a routing system characterised by high discharges. Misquoting Shakira “the sediments don’t lie”; they record a history of sustained water transport and habitability on early Mars.

A beneficial outcome of ExoMars Rosalind Franklin Rover (ERFR) 1,2 landing site selection process has been the spinout science from detailed studies of parts of Mars that had not previously been examined in detail. Here, we present the geological description of Aram Dorsum3 (Fig. 1), a well-preserved, flat‐topped, branching, ~85 km long and ~ 1 km wide ridge system in western Arabia Terra that was a ‘top 3’ candidate site during ERFR site selection.Fig 1. CTX Mosaic Showing Aram Dorsum (sinuous ridge running top right to lower left).We use morphostratigraphic mapping of the Aram Dorsum ridge and surrounding area, and detailed morphological observations, to propose a consistent working hypothesis for the geological history of the region. Our observations and mapping reveal Aram Dorsum to be the sedimentary deposits of an extensive aggradational fluvial channel belt system, now preserved in positive relief by differential erosion. The existing ridge was once a large river channel belt set in extensive flood plains, many of which are still preserved.Aram Dorsum is part of a wider set of similar inverted channels found across Arabia Terra4,5, and thus was probably part of a regional fluvial system, demonstrating movement of water and sediment across large distances. Furthermore, several smaller palaeochannel belts feed into the Aram Dorsum ridge from within the local regions, and their setting and network pattern suggest a distributed and local source of water. Aram Dorsum therefore appears to record both regionally and locally distributed sources of water.Combining mapping with HiRISE6 and CTX7 Digital Elevation Model data reveals that the Aram Dorsum alluvial succession is up to 60 m thick, suggesting a formation time of 105 to 107 years by analogy to Earth8. Correlating our observations with previous regional‐scale mapping9 shows that Aram Dorsum formed in the mid‐Noachian, a result supported by impact crater size frequency distribution measurements.The Aram Dorsum formation comprises a succession of what are, by analogy with terrestrial fluvial systems, probably coarse‐grained fluvial channel belt sandstones and finer‐grained overbank deposits. The vertical thickness of alluvial succession equates to several cubic kilometres of fluvial sediments in this study region alone. That other inverted channels elsewhere in Arabia Terra4,5,10 are similar in morphology and scale suggest that similar thicknesses and volumes of mid‐Noachian to late‐Noachian fluvial sediments may be extensive and common in the wider region.Aram Dorsum was an extensive long-lived fluvial system with distributed sources. This suggests that local and regional precipitation (either as rain or as seasonal or repeated snow melt) was the source of water. That Aram Dorsum is one of several similar systems suggests that precipitation was widespread across western Arabia Terra during this period. In contrast, Aram Dorsum's low elevation and distance from the majority of the Valley Networks, argues against the source of water being melting of a distant, high‐altitude, ice sheet, or ice cap11,12. Similarly, the aggradational fluvial depositional setting and the scale of the system do not suggest deposition from multiple short‐lived fluvial flows, as might have occurred due to impact cratering or catastrophic volcanic outgassing temporarily altering the climate12–15. We conclude that Aram Dorsum is one of the oldest fluvial systems described on Mars and indicates climatic conditions that sustained surface river flows on early Mars.References cited